This invention relates to carbon fibers and to a method for producing same, and particularly to high strength, melt spun carbon fibers and method for producing same.
There are many commercial uses for fibers which are high in strength and light in weight. Carbon/graphite (C/G) fibers exhibit such high strength and light weight mechanical properties.
The mechanical properties of C/G fibers depend upon how well their structure resembles the anisotropic structure of an ideal, i.e., perfect, graphite crystal. As shown schematically in FIG. 3, the three dimensional lattice structure of an ideal graphite crystal is basically a network of hexagonal crystal planes stacked one on top of the other with an orientation such that within each layer covalent carbon-carbon bonds link individual graphite crystals together in the plane. These strong bonds give graphite its high strength characteristics in the direction parallel to these planes. Each layer of hexagonal crystal planes is perfectly parallel to its adjacent planes. Because these planes are perfectly parallel to one another, the interlayer spacing is very small, and consequently the ideal graphite crystal has a very high density. The closeness of these parallel planes gives graphite a high stiffness characteristic. A perfect crystal has a theoretical tensile modulus of elasticity of 146 million pounds per square inch (msi), and a theoretical ultimate tensile strength of 15 msi.
Commercially produced C/G fibers differ from the perfect crystals of an ideal graphite lattice structure due to both surface and internal flaws and in the lesser amount of preferred orientation along the fiber axis, which is in the direction parallel to the hexagonal crystal planes. Structural flaws affect the ultimate tensile strength, and the degree of preferred orientation along the fiber axis affects the tensile modulus of elasticity.
Carbon/graphite fibers have been produced from a number of different precursor materials. One such material is polyacrylonitrile (PAN), which is described as an atactic linear polymer whose fibril 3-D network tends to form an irregular helix structure, which is generally designated in FIG. 1 by the numeral 50.
A typical process for producing PAN-based C/G fibers is shown schematically in FIG. 1. Typically, the as-spun fiber is obtained by wet spinning PAN or its copolymers into a coagulation bath. The purpose of using a copolymerized precursor is to lower the glass transition temperature, thereby allowing the as-spun fiber to be stretched in liquids which boil at lower temperatures. The as-spun helical fiber is stretched to better orient the polymer molecules along the fiber axis. It is thought that oxidation of the stretched fiber maintains the preferred orientation along the fiber axis by cyclization of the nitrile groups as shown in FIG. 1. Suggested temperatures for oxidation are 220.degree.-270.degree. C. for up to seven hours. Most of the non-carbon elements are driven off in gaseous form during the carbonization step, which occurs in an inert atmosphere between 1000.degree. and 1500.degree. C. Stretching the fiber during carbonization also improves the strength and stiffness of PAN-based carbon fibers. A further heat treatment step can be performed at temperatures between 1800.degree. and 2500.degree. C. for less than one hour to purify and provide a higher degree of preferred orientation of the 3-D turbostratic structure.
The modulus of elasticity of PAN-based C/G fiber increases with heat treatment temperature, but the tensile strength reaches a maximum value of approximately 450 ksi at a temperature of approximately 1600.degree. C. Surface flaws in the as-spun PAN-based fiber may be retained throughout the entire process and limit fiber strength. Internal flaws caused by voids left by rapidly evolving gasses may occur during heat treatment and cause a decrease in tensile strength with higher temperatures. Moreover, the stretching required to obtain the desired strength characteristics is time-consuming and expensive in commercial production.
Since PAN will thermally decompose prior to melting, a solution of PAN in a solvent such as dimethyl formamide is normally spun into a filament using either a "wet" solution spinning technique, as described above, or a "dry" solution spinning technique. In both wet and dry spinning, the solvent must diffuse through the filament and then evaporate into the spinning chamber (dry spinning) or enter the coagulating bath solution (wet spinning). If the rate of evaporation of the solvent (or the rate of loss of solvent into the coagulating bath) is less than the rate of diffusion of the solvent through the PAN filament, the filament will dry uniformly and the filament will have a circular cross-section. However, if the rate of loss of solvent at the filament surface is greater than the rate of diffusion of solvent through the filament, then the surface of the filament will harden faster than the core, and a fiber having a collapsed, dogbone-shaped cross-sectional area will result. Thus, it is this balance between mass transfer away from the fiber and diffusion within the fiber which normally governs the shape of the fiber's cross sectional area in PAN spinning processes. The precipitation process required to produce a PAN fiber limits the possible non-circular cross-sections which can be produced and stably controlled in a commercial process.
A PAN-based carbon fiber having a dogbone-shaped cross-section is observed to be lower in strength than PAN-based fibers of circular cross-section. PAN-based fibers having a trilobal cross-section also is observed to be weaker than PAN-based fibers of circular cross-section. The strength of PAN-based fiber of circular cross-section decreases with higher carbonizing temperature. However, there is some evidence in the literature that dogbone-shaped PAN-based fiber becomes higher in strength with higher carbonizing temperature.
Pitch, whether natural in origin, such as coal tar or petroleum pitch, or synthetic in origin, such as specially prepared polyvinylchloride (PVC), has been used as a precursor for producing a melt spun C/G fiber. Pitch, a graphitizable substance, is a collection of hydrocarbons ranging from low molecular weight paraffins to high molecular weight large aromatics. A graphitizable substance has been defined as one which fuses or becomes plastically deformed during heat treatment. According to this definition, rayon-based and PAN-based C/G fibers are not graphitizable. While they may set up in a turbostratic configuration, rayon and PAN are incapable of forming the characteristic three dimensional structure of graphite.
As discussed in this patent application, graphite fibers are considered to be those fibers which have been heat-treated above 1700.degree. C. and have a carbon content of at least 99 percent. The typical graphite structure is shown in FIG. 3. Carbon fibers are those fibers which have been heat-treated below 1700.degree. C. and have a carbon content of between 80 and 95 percent.
It has been reported that upon heating graphitizable substances such as pitch materials, the original material melts or fuses to form an isotropic pitch-like mass. As heating continues, spherical bodies begin to form. The spherical bodies are of an anisotropic liquid crystalline nature as viewed under polarized light. These spheres continue to grow and coalesce until a dense continuous anisotropic phase forms, which phase has been termed the "mesophase." Thus, the mesophase is the intermediate phase or liquid crystalline region between the isotropic pitch and the semi-coke obtainable at higher temperatures.
U.S. Pat. No. 4,208,267 discloses a method for producing mesophase pitch-based C/G fibers in which a nearly 100 percent mesophase pitch precursor is melt spun. This method is illustrated schematically in FIG. 2. The nearly 100 percent mesophase precursor is prepared by converting a solvent-insoluble fraction of isotropic pitch into an anisotropic pitch containing between 75 and 100 percent mesophase by heating to between 230.degree. to 400.degree. C. for less than ten minutes. For the most part, it is the large aromatics which convert to the mesophase upon heating. The solvent-insoluble fraction is pelletized as a solid and then melt spun through a conventional screw extruder at spin temperatures of between 360.degree. and 370.degree. C. to produce a fiber filament of circular cross-section. Typical viscosities for the mesophase precursor at such spinning temperatures range between 200 and 700 poise.
If the as-spun circular fibers produced from the mesophase were immediately subjected to carbonizing temperatures, the fibers would degrade and lose their anisotropic molecular orientation. To avoid loss of orientation, the as-spun fibers are thermoset at 200.degree. to 350.degree. C. in an oxygen atmosphere. After this oxidation step, carbonization/graphitization is accomplished in a horizontal graphite resistance furnace at temperatures between 1000.degree. and 2000.degree. C. under a nitrogen atmosphere.
It has been thought that the orientation which is imparted to the mesophase during spinning gives rise to the graphitic orientation developed in the fiber during the carbonizing steps. As the molecularly random mesophase precursor flows through the spinneret capillary, a certain amount of order is produced such that the liquid crystals preferentially orient themselves along the longitudinal axis of the fiber. Accordingly, the costly Process of high tension heat treatment is not needed by mesophase pitch-based C/G fibers to induce preferred alignment.
Commercial producers of synthetic fibers have produced non-circular synthetic fibers from melt spun polymers, such as polyester, nylon and polypropylene, for about 20 years. The extrusion process is identical to the one used to produce circular synthetic fibers, except that spinnerets with non-circular capillaries are used rather than ones with circular capillaries.
Polymers have a relatively large range of temperatures over which the viscosity of the polymer is suitable for producing a melt spun fiber, whether circular or non-circular in cross-section. A polymer such as polystyrene shrinks during the draw-down process of melt spinning under typical commercial conditions, from a diameter of about 700 microns to a final diameter of about 40 microns over a distance of about 40 millimeters. This distance is sometimes referred to as the quench distance and is a critical parameter in obtaining a non-circular polymer fiber.
For many materials, surface tension is the single most important obstacle to overcome in melt spinning non-circular fibers. For example, the high surface tension of glass has prevented commercial production of non-circular glass fibers. Polymers, on the other hand, more readily lend themselves to being spun into a C-shaped or annular fiber because polymers have a relatively low surface tension.
Several factors reduce the likelihood that C-shaped or hollow carbon fibers can be produced by melt spinning an anisotropic precursor such as mesophase pitch. First, anisotropic precursors have a surface tension between that of glass and that of polymers. In addition, the quench distance for a circular carbon fiber produced from an anisotropic precursor is approximately 4 mm over which a 200 micron diameter is drawn down to a twelve micron diameter. Third, the viscosity of an anisotropic precursor is far more temperature dependent than the viscosity of polymers.